1. Field of the Invention
The invention relates to a power supply and scanning line deflection circuit for a picture display device, said power supply circuit comprising, connected between the terminals of a first voltage source, the series arrangement of a first switch switchable at line frequency and a first inductance, the first inductance forming part of a first resonant network during the cut-off period of the first switch, the line deflection circuit comprises a line deflection coil and, connected between the terminals of a second voltage source, the series arrangement of a second switch switchable at line frequency and a second inductance, the line deflection coil and the second inductance forming part of a second resonant network during the cut-off period of the second switch and which substantially coincides with the retrace period. Connected to a transformer winding coupled to one of the inductances is rectifying circuit including a rectifier for rectifying the pulses present across the transformer winding during the cut-off period of the switch which is in series with the relevant inductance, for generating a supply voltage for a varying load.
2. Description of the Prior Art
Such a circuit comprising a power supply circuit and a line deflection circuit is disclosed in U.S. Pat. No. 3,999,102. Therein; the supply voltage is stabilised against variations of the first supply voltage which is, for example, derived from the power mains, but keeping the supply voltage substantially constant for the varying load causes problems, particularly if the variations are fast and relatively large. This is caused by the fact that the rectifying circuit can be considered to be a source having a rather higher internal impedance as energy is stored in the inductance to which the transformer winding is coupled, in a time interval preceding the interval in which the energy is consumed by the load, which energy cannot vary rapidly, and as the transformer provided by the said inductance and the winding has a rather large leakage inductance. Because of this high internal impedance the rectified voltage decreases when the load increases, which may have disadvantageous results. If, for example this voltage is the high voltage for the final anode of a picture display tube, the load being formed by the beam current within the tube, then the high voltage assumes a low value in those portions of the displayed picture which have a high luminance, these portions then being of a larger size on display than portions having a lower luminance, this phenomenon being the co-called "picture breathing", which entails a loss of picture quality.
The invention has for its object to provide a power supply to scanning line deflection circuit of the above-mentioned type in which the internal impedance of the supply voltage source for the variable load is reduced using only few additional means, such that a considerable variation of the load does not produce a large variation of the value of the voltage applied thereto and to that end the picture display device according to the invention is characterized in that it further comprises a connection for connecting, at least during the conduction period of the rectifier, a point of the transformer winding or of a winding coupled thereto to a point of the inductance not coupled to the transformer winding or of a further winding coupled to the said inductance, this connection being substantially currentless in the non-loaded state of the rectifying circuit.